Extractive distillation with salt solutions



Sept. 30, 1952 c. E. MORRELL ET AL EXTRACTIVE DISTILLATION WITH SALT SOLUTIONS 2 SHEETS-SHEET l Filed Nov. 4, 1947 momma M. mex m` c/ MNE mw. r mf wy 2 l a; pw @Fo W2 EL /2 m2 m s E s FTQ-l @29d .31346 ozosw. om Boz om.. oh Ow Om O v Om GN Q O 2 SHEETS-SHEET 2 C. E. MORRELL EVAL NNQLWUQ rSOw, WSONGGQ EXTRACTIVE DISTILLATION WITH SALT SOLUTIONS Sept. 30, 1952 Filed Nov. 4, 1947 N .HN

Patented S'ept. 30, 1952 UNITED STATE EX'raAc'rlvE Drs'rILLA'rIoN wrm SOLUTIONS SALTv Charles E. MorrelLvWestiield, vN. J., and Edwin R. l

Gilliland, Arlington, Mass., assignors to Standard Oil Development Company, a `corporation y -of Delaware Application November 4, 1947*'S-erialvNo. 783,989I Y 1 This invention vrelates -to yanimproved` method for separati-ng vaqueous mixtures of oxyorganic compounds; particular-ly thosey whichv form closebolling azeotropeswith water.- The invention has particular application to the separation of aque` ous solutions of low molecular weight oxy organic compounds having' less than about'` 9 carbon atoms vper molecule 'such as the alcohols of 2 or more lcarbon atoms per molecule and Vthe carbonyl-containing compounds (aldehydes, ketones, esters, acetals, ketals, ethers, etc.) 'of about '2 to 8 carbon atoms per molecule. AThe-separation is accomplished by distillingsuch mixtures and passing the vaporsl countercur'rentfto a large vvolume'of an aqueous -reflux stream `'containing an electrolyte dissolved therein.

rlhere is described in a -copending application, S. N. 724,840, iiled January 28, 1947,'fby Gilliland, Morrell, ICarlson and lRobertson, y'o'f which the present -application is a` continuation-impart, now S. Patent 2,551,593,aprocess for separating such aqueous mixtures by distillation in which the vapors are passed countercurrent to a refluxstream containing a high proportion of water. For example, a mixture 'offfethanol and isopropanol is distilled in a tower while passing downwardly through the tower suflicient water to maintain a concentration of more than 60, and preferably 90 to 97 mol percent of lwater on the plates of the tower. Under-such distillation conditions the volatility of isopropyl alcohol is increased relative to that of ethyl alcohol and substantial separation is obtained with isopropyl alcohol being concentrated inthe distillate and ethyl alcohol being concentratedin the tower bottoms, both concentrations being relative to the total alcohol basis in the feed. Bothztheisopropyl alcohol distillate and theethyl alcohol bottoms are obtained as aqueous solutions, :from which the alcohols may be lreadily distilledby ordinary methods. OperatingI in this manner, ethyl alcohol and isopropyl alcohol-can lbaseparated to any extent practically desirable so that either alcohol can be obtained'in pure 'form with' only traces (0.1% and even less) of the other. The process of this application is also applicable to the separation of one or both of these alcohols 14 Claims. (Cl. 202-39.5)

` from higher-boiling alcohols, from aldehydes, ke-

tones and their condensation products such as' the .acetals and ketals, etc.

It has been found that this process is improved bythe addition `of an electrolyteto the water reiiux in lthedistillation tower in which the separation .of alcohols, etc., is carried out. The operation! is votherwise the same asV described in S. N. 7.2%840 mentioned above. The addition of such electrolytes has been `found to 'change the relative volatility, or r,alplfia, of the 4compounds undergoing separations as described above. Such electrolytes include ythe water-soluble lsalts which are inert Ato or react very slowly under the conditions of operation finboiling dilute aqueous solutions with the oxy :organic compounds undergoingsep -v arationV and'which are substantially vstable under such conditions. Anytsalts meeting these requirements may be used -in accordancewith this invention, including those-'which form weak, reversiblecomplexes with one orv more of the .oxy organic compounds.A 'Suitable salts include those of alkali and alkaline earth metalaammonium,"

magnesium, copper, iron, cobalt, nickel and the like with acid'fradicals including" vthe halides, phosphates, .sulphates, carbonates, nitrates, boratespsilicates, -chromates and the like. Inan even broader sense vit is intended to include` such salts o f acids derived ...from 'the A`various oxides (real orzhypothetical) `of boron, carbon, nitrogen, sulfurJ phosphorous, silicon, chlorine, bromine, iodine and-chromium; Similar salts of organic acids ,mayfalso lbe usedafsuchv as those of acetic, benzoic, c itric,fformic and lactic Yacids and the like. All ,these saltsinclude those in which only par-t of the .acidic -component 'has `been f neutralized, commonly known asmetal acid '.salt's; e. gr., the Vsodiu-macid phosphates.` It is recognized that -thesalts'rnentioned above vary widely .as regards water solubility,` stability ,in boiling VKsalt solutions,` `reactivity 'toward some 1 organic .compounds and tendency `to attackmaterials of construction auf, variousA types-., asl a 'resultvv of these diiferences, the salts shouldlbe selectedwith regard-,to the--chemical nature ofthe -oxy Aorganic compounds to :be separated, the distillation Acon- A lditions of temperature and pressure, andthe coriosivity of the materials of construction used in the tower and other equipment. The following general principles should be considered in selecting appropriate salts for any separations desired.

The water solubility of the salt is of considerable importance. In general, it is desired that it be suiciently water-soluble to produce a finite lowering of the vapor pressure of water in a saturated solution, since one of the purposes of this invention is to provide .for lower water vapor content of the gas phase in the extractive distillation zone. This solubility requirement renders such compounds as the alkaline earth normal sulfates and phosphates Vof little' interest.' It is also desirable that the Water solubility of the salt be sufficiently high for it to dissolve in the mixture of water and oxy organic compounds to be tals.

separated, to an extent suflicientto produce two Y phases, one rich in the oxy organic compounds However, in the and the other rich in water. operation of the process of this invention; it is most desirable to conduct the extractive distil-A f lation operation in such a manner that such salting-out phenomena do not occur and that for the lowest temperature in the extractive distillation zone, the salt lconcentration is just below the value necessaryto produce phase separation. In other words, the salt concentration should be low enough so that only one liquid phase is present on the distillation tower plates. Salt concentrations appreciably lower than this value may of course be used, but in general do not provide as eiective results as regards enhancement of separation coeiiicients of the oxy organic materials being distilled and as regards minimizing the Water content of the vapor in the extractive distillation zone. In the separation of the higher molecular Weight oxy organic compounds, it is desirable to use those salts which exhibit high molal solubilities in water without producing insolubility of the oxyA organic compounds. The higher molecular weight oxy vorganic compounds in the range described above are, of course, of limited solubility in water. Hence, permissible salt concentrations are generally lower than those in the case of the lower oxy organic compounds. The question of stability of the salt is also of considerable importance, since in general in extractive distillation operations the salt solution must be subjected to boiling both for the purpose of obtaining vaporization of the oxy organic compounds and for'adjusting the concentration of the resulting solution obtained as distillation residue. In general, salts which decompose by hydrolysis to formV precipitates or which change structure as the result of evolution of their acidic or basic components as gases from boiling aqueous solutions, are not very suitable for use in this operation. Examples of this type are the bicarbonates of the alkali metals which decompose with evolution of carbon`dioxide to form precipitates. Other illustrations are the bisultes of the alkali metals which evolve acid gases under boiling conditions. Freedom from destructive and irreversible reactivity with the oxy organic compounds present in the extractive distillation zone is obviously of considerable importance. In this respect the salts may have two more or less'dependent effects. In the first place, they may react directly with certain of the oxygenated compounds such as the aldehydes or ketones or acids to produce other materials. In thisconnection, when dealing with mixtures of oxy organic Vcompounds alcohols, aldehydes and ketones.

mote reactions between various oxy organic compounds. A typical reaction of this type is that of aldol condensation, which may involve the interaction of one or more molecules of a ketone or the reaction of one or more molecules of an aldehyde or the interaction of aldehydes and ketones. In certain other Acases salts may also promote interaction of alcohols and acids to form esters or of alcohols and aldehydes to form ace- In general, itis desired to eliminate all such combination reactions of the various oxy Lorganic compounds in the extractive distillation zone.

of theircomponents and greatly complicates the separations involved. Thus it is often desirable to remove acidic oxy organic compounds from the crude mixtures by solvent extraction, preliminary distillation or neutralization with aqueous alkali prior to the extractive distillation, when the mixture contains acids and alcohols capable of esterilcation during the distillation.

As applied to the separation of oxy organic compounds from, the Fischer synthesis (or hydrocarbon synthesis) products, it is considered that the tendency of the carbonyls (aldehydes and ketones) to undergo aldolization is one of the more important questions limiting Athe type of salt to be used. It is known that aldol reactions exhibit lowest reaction rates in pH ranges close to 7both higher and lower pH solutions tending to promote these reactions, the aldehydes being the most reactive and sensitive to change inA pH-,rom 7. lIn the case of oxyorganic compound mixtures containing only alcohols and the less reactive ketones, it is possible -to use a fairly wide range of pH on both sides of the neutralization point, fromabout 3.0 to about 10.0, while with aqueous solutions containing only mixtures of alcohols, a very much wider range of pH can n be used without encountering reaction difficulties.

While it is generally desirable in this invention to avoid reactions which are primarily of a condensation type and which result in formation of compounds of higher boiling points than those of the original oxy organic compounds being separated; hydrolytic reactions are permissible andin many cases are desirable. Thus, with feed mixtures containing esters, acetals or ketals, such compounds `may be broken down to their elementary building blocks such as the acids, The alcoholic constituents thus formed may then be recovered along with the other alcohols and the carbonyl products along with the uncombined carbonyls originally present inthe feed to the extractive distillation.

In working in steel equipment, corrosion of the steel can most readily be avoided by maintaining the salt solutions at a pH of about 8 or higher.

In view of the above, it is possible to point out the preferred types of salts for use in certain special cases and especially for the separation of oxy vorganic compounds from the aqueous layer obtained in Fischer synthesis reactions. As stated above, it is desirable, in order to avoid corrosion of steel, to maintain a pH of 8 or higher. Furthermore, in order to avoid aldoliza- Formation of these condensation vproducts results in Wide variations of the volatilities tion in such mixtures, it is desired that thispI-l` range be limited to a maximum value of about group which especially meet these requirements l are' those obtained by partial neutralization of the`Y phosphoric acids. The neutralization of o'r-tli'o'l phosphoric acid with approximately 2 molecules of .sodium hydroxide per molecule of acidg'ives alighly satisfactory salt (NazHPO'i) iorfthis purpose. Sodium sulfate has certain desirable properties in that it'gives a' pH rangev of about '7 and does not greatly promote aldolization. Boric acid'salts also-.are useful 'and have the property that the pH may be varied by `partial neutralization of the boric acid. Sodium carbonate is yeiiective when used in proper conv centrations in preventing corrosion and is appli cable in cases where its relatively high pH does not cause4 trouble' from aldolization, i'. e., when distilli'ng mixtures of oxy organic. compounds relatively free of aldehydes and ketones.

.Mixtures 'of any of the salts described herein, which do not react to iormpre'cipitates or evolve gaseous products under the operating conditions,

may also be used to provide both control 'offpH andenhan'cement in the relative volatilities voi the materials being separated.

'The aqueous salt solutions can be used for extractive'xdistillation in accordance lwith the present invention, to conduct separations of aqueous mixtures containing the following classes of'organlc lcompounds: acids, alcohols, ketones, aldehydes;'-esters, acetals, ketals, ethers and hydrocarbons.. The separations canbe. carried outin theorderabove indicated, the compounds of one or more classes being separated from those ofthe latter class or classes in the series.. Similarly,

mixtures of oxy organic compounds in the same class v'can also be separated by extractiva distillation with aqueous salt solutions according to this invention. `In' the case of mixtures ci isomers of the. same class, the volatility of the` isomer of higher order is increased relative to the volatility of an isomer of lower order. For example, in the case of alcohols, four different crderswmay be mentioned: y(l) primary normal alcohols, (2) primaryibranche'd chain or iso alcohols, (3)1 Js'econdary'alcohols and (4) tertiary alcohols. Isomers oi" one or more. orders of such compounds are thus increased "in volatility relative' toy iso` descending through the distillation zone.. Ingen eral, increasing the concentration of-water in this liquid phase increases vthe change in relative volatility of the compounds being distilled, but it is recognized that this invention has practical limits where the compounds to be separated are substantially insoluble in water or are 'substantially involatile. Also, when the compound whose volatility is increased in this process normally boils, when pure or as an aqueous az'eotrope', at a temperature lower than thatV of the other come; pound or compounds from which it is to, be separated, Vthe use `of very high aqueouss'olvent concentrations in the liquid phase of the distillation zone inay be necessary to reverse the volatili-v ties of the compoundsto be separated. -.Thus,1in dealing lwith such mixtures it is important for. practical purposes that the organic components, which require a reversal in volatility for separation'ir'lr the present process, distill as pure compounds or azectrop'es within a fairly narrow boiling lrange in lorder to permit' economic tower sizes and"w'ater `concentrations in the reilux.

Generally the molal concentration of solvent in mers oil-the preceding, lower orders. In the case of homologous mixtures of oxyV organic compoundsl (of the same class but differing in molecular weight), the volatility of thecor'npound of higher molecular weight is increased relative to the volatility of the lower molecular weight compound. In the case of separations of compounds fallingin the-different classes indicated above, the volatility of the'compounds in the last named classes is increased` relative to the volatility of thev compounds inthe preceding classes inthe series. Thus, the separations can be made between individual compounds in a class or between classes of compounds in the sequence namedl above. With more complex' i mixtures,

to the present invention by distillingmixtures of such compounds'in a reflux column in which the reilux liquid'is an aqueous salt solution 'which is used'. in such proportions as to provide a high molal concentration of water in' the liquid phase Ythe classes indicated above.

the liquid reflux in the distillation zone isabove 50 to 60 with even higher concentrations causing astilllarger change' in' relative volatilities of vthe compoundsbeing distilled.' For the` n'1for'el diiicult separations discussed', above, solvent con; centrationsof to 97 %,98% "and even higher may. be' used. A` lAs the solvent' concentration alppreaches infinity the selectivity still increases', but the eiciencyli's' lowered' on account of the' relatively small quantities of the organic'cornpounds involved.' This is not` a"serious limitation ofthe present'jinvention, for' compounds having Widelyy diierent boiling points can be readily separated b'y ordinary distillationinethods, the presentl invention being' especially applicable to those mixtures having boiling points so close toe getherthat efcient separation' by ordinary distillation methods is'not practical. 'An example of such a mixture' is the aqueous mixture' of ethyl alcohol (aqueous azeotrope, B. P. 78.1 C.) and isopropyl alcohol (aqueous azeotrope, B. P. 80.2 CJ). y Such alcohcls'are so close boiling that it is not practical Ato purifythem'by ordinary distillation of their mixture, but in the present process the normally higher `boiling isopropyl alcohol can bewdistilled overhead from the ethyl alcohol.- Other examples include narrow boiling ra n ge mixtures` containing thev alcohols, Oxygenated organic ,compounds and hydrocarbons of Wherethe crude mixtures obtainedV boil' over a wide' range, it is quite feasible toobtainnarrow boiling'range mixtures desirable orseparation by the present 'invention by conventional distillati nf i.

Some of the above-described mixtures are obtained by an olefin hydration reactiome. g., when `'a mixture of ethylene and propylene is absorbed in sulfuric acid, diluted, hydrolyzed, and a result ing aqueous` alcohol mixture is stripped out. Another important source of such mixtures'is the Fischer synthesis hydrogenaton of carbon monoxide', with orfwithou't added ole'iins, especially Whenthe aqueous layer product formed contains not only lower primary and secondary alcohols but alsovarious ketones, aldehydes, ethers, acetals, esters, carboxylic acids, and/certain tertiary alcohols." Still another source is in the products of hydrocarbon oxidation where both oiland water layers arek obtained, both containing oxyf TABLE I Ethanol out Compounds l Anhydous Agtgs n B. Pt., C. B. Pt. D

Such fractions, distilling in the range between about 20 and 97 C. and containing about' 20 to 40 volume per cent of Water, are obtained by distillation of the Water layer of the product of the Fischer synthesis process for hydrogenation of carbon monoxide.

"'InV such crude ethanol cuts such as those described above, the kinds-and quantities of the components Vary greatly but the'major c omponents are generallyv ethylv alcohol; .isopropyl alcohol, methylethyl `ketone and n-'butyraldehyde. .Repeated fractional distillations of such ethanol cuts were found to beof no avail for obtaining isolation of pure ethyl alcohol or pure isopropyl alcohol. The diiculties encountered can be appreciated by reference to Table I which shows the overlapping of the 'boiling points and by considering that these compounds formk additionalrazeotropes with one another. With the present `invention it was found possible to effect the critical separations necessary in recovering the pure .alcohols freed of the other substances normally boiling in the same narrow range, even vthough the contaminating substances Vhave `relatively lower and higher boiling points.

- Narrow-boiling range mixtures 'which maybe obtained' by the ordinary distillation processes from such crude aqueous products areas follows:

vNarrow-boiling range alcoholmirtrcs u Com onents u Azeotrope Gfp, p .f van', 1 s. am.

Ethyiaicohoi.-. Q 78.5 I 78.1 1""1 {Isopropyl Alcohol V 82.3 80.4

I C0 0...-.. H'" {sop opy l79.6 73.6 @78.5 78.1 111 i r2.3 80.4 j lt-Butyi A1coh01 1 sa@ l 79-9 :The narrow-boiling range Amixture may bea binary `or ytertiary mixtura. as` in Uthe groups shown, but, generally, the crude mixtures contain additional oxygenated organic compounds, which do not interfere with the basic operation otthis invention in isolating the Vprincipal..alcohol cornponents of the mixtures. l 1t is tope noted that the aqueous azeotropes of thev alcohols in these mixtures have vboilinLy points Whichdiffer bygless l than vecentigrade degrees.V It is` generally'de-f sirable to use such relatively narrowcut mixtures boiling over a range of not more than.5 or 10"v C. in the present process, although fractions boiling over a much Wider range of 15 C. or more may also be used, as described below.

V.Examples generally of specific mixtures which can be separated by extractive distillation with aqueous salt solutions according to this invention are as follows:

. boiling range. The less Volatile compound which is obtained in the extractive ydistillationresidue in the followingvspecic separations, is named first: y

(l) Ethanol from isopropanol.

(2) Ethanol from isopropanol and the butanols. l Y (3) Ethanol from visopropanol, the butanols and the amyl alcohols.v l i (4) Ethanol (with methanol, if present) from all or any one or anycornbinationof the aliphatic alcohols of 3 to 6 carbon atoms per molecule.

(5) Ethanol from methyl ethyl ketone and higher ketones such as methyl normal propyl ketone and methyl normal butyl ketone.

(6) Ethanol from butyraldehyde and the higher aldehydes.

(7) Ethanol from ethyl acetate, .acetal and the v ketals.

Methanol, if present in any of the initial mixtures (1) to (7) described above, is also obtained in the bottoms.

(8) Isopropanol from the butanols.

(9) Isopropanol `from .methyl ethyl ketone'.- (10) N-propanol (with ethanol andmethanol, if present) from all or any one or any combination ofthe aliphatic alcohols of 4 6 carbon. atoms per molecule and from isopropanol.

'(11) VN-butanol from'other I.C4 alcohols (secondary butanol, tertiary butanol, isobutanol) and all aliphatic alcohols of 5. and .6 carbon' atoms per molecule. L .Y f

(12) N-butanol from the Cs aliphatic alcohols. (13) Isopropanclfrom all higher alcohols and specifically from rall higher secondary and -isomolecule. (14).. Secondary alcohols of Sto-5 carbon atoms per. molecule from all aliphatic alooholsof higher molecular Yweight of 4 to 6 carbon atoms per molecule'. f (l5) Isoalkyl alcohols from higher molecular aliphatic alcohols of 4 to 6 Icarbon atoms per weight isoalkyl alcohols of 3 to 6 carbon atoms per The drawing (Figure l1) is a diagrammatical illustration in partial `sectional elevation of suitable equpxnent for carrying out the processrof this invention and indicates the flow of materials. It will ber described'in'relation to va'specic ex.- ampleof the application offthis process to` the separation of pure ethanol from a crude ethanol fraction. While. the process' is'applicable directly tolthel separation ofethanol .from the mixtures described in Tables I and `II,`rfor simplcity,`it will be described with a feed cut ofy the following composition:

. This crude alcohol mixture is supplied byline I to the mid portion of a ldistillation tower/,2 which may be supplied rwith suitable plates Vor packing for efficient countercurrent contactv of liquid and vapor and' whichl contains the equivalent of about V30 plates belowy the feed, and 30 plates between the feed and a;solvent supply 4line 3. .A water rectifying section 4 containing about plates may also. be provided above'the solvent supply line and is,v of valuein reducing the Water content of the distillate .'to composition approaching any azeotropes whicharezobtained. f Y The top of the tower may also be supplied with any suitable methods for returning refiux thereto from partial or total distillate condensers such as )the cooling condenser `5, condensate receiver 6, reflux return line 'l and distillate withdrawal line 8. The bottom of the tower is also supplied with suitable heating means for reboiling the tower bottoms such as the closed steam heater 9 in the tower bottoms line I0 provided with a heated, bottoms vapor return line II. i

The tower bottoms are passed by line I2 to a second distillation column I3 which may befof any suitable design for separating volatile prod'- ucts from liquid under fairly good fractionating conditions. Its function inA dealing with the specific feed stock described above is to separate an ethyl alcohol-water azeotrope, with or without additional water as may be desired, from The process may beoperated with the feed composition described above in the,followingk manner.

Example 1 'Anaqlleous zSaIt solution containing-9.73 mol I perlvceni.'v sodium acetate and the balance water, is` .suppliedy at `a rate ofy 3680 gallons: per'hour and'at a, temperature of 190 F. in line 3 to tower 2 towhichthe crude alcohol feed mixture is supplied byline I at a rate of 650 gallons per hour; 'The' reboiler and `reflux condenser are operated to provide a reflux ratio of.15'jvol- .umes 'per volume of 'feed in' section 4,"thus giving" a distillate containing about 11% Water, whicl approaches the water content of'v the yaqueous'"azeotrope's taken overhead. The sol- 'vent' returned in line v3 is in such quantity to provide a "proportion of solvent to oxyorganicf compounds on the plates throughout tower 2 be-.J low the solvent line 3 of about 85 to 95 ymol per cent solvent -(sodium acetatev plus water). Under-'these `conditions-in operating for the production offethyl alcohol-of high purity, a small proportionvrof ethyl alcohol maybe taken over- .head in .zorderwto-insure the thorough removal ofisopropylzalcohol and methyl ethyl ketone at thisfpoint;.1thus"providing a distillate in line 8 Methyl ethylketone 40.4 vIsopropyl alcohol- 40.4

Ethyl alcohol 8.1 y Waiter lr.......'. y '\11.1

the bottoms leaving tower 2. Tower I3 is provided with any suitable partial or complete distillate condenser for supplying reflux such as the condenser I4, condensate receiver I5, reflux return line I6 and distillate product withdrawal fevent that towers 2 and Hare-operated at dif- 1, ferent pressures. Since it `is generally .,un

desirable to withdraw solvent containing an electrolyte or salt as used in accordance with Athis invention, and yet the solventrbottoms I0 process vthe 'solvent f of the following composition: i

.. Molpernt yThe bottoms from tower 2l 'in line I2 are thus a dilute solution'of ethyl alcohol in `aqueous sodium acetate free of all but minute traces of higher alcohols and ketones. Under these conditions towerl I3 may be operated as a simple 'ethanol still" tol produce the aqueous' ethanol azeotrop'ev (96 volume per cent ethanol) ,asr the distillate invv line I1. This ethyl alcohol is of extremely high purity andl goododor and contains `only-.traces not exceeding 0.1% `of other organic compounds.

A: feed mixture 'containing 80 mol percent ethanol, 4 per cent methyl ethyl ketone, 4 per cent isopropanol and 12 per cent water, was sup- A* plied at 30 C. at a rate, of 106 volumes per hour to the mid-point of a packedcolumn equivalent to theoretical plates equipped with a solvent f supply line yand a vapor reflux condenser at vthe top and heating means at the bottom.

' An aqueous solution of 33 weight per cent sodium acetate was supplied as solvent at a temperature of 94 C. and a rate of 1940 volumes per from tower 2 may tend to become diluted with Water from the feed mixture, it is desirable to Y.

provide means for withdrawingv this dilution water from the solvent before its return in line ing the excess water supplied with the feed.

65.. 3. Thismay be done by operating towers and/or I3 to take a distillate fraction contain-1 Also, by designing heater I8 withsuitable liquid-` vapor separators so that only vapor is returned in line 20, any desired portion ofA the excessl water supplied with the-feed may be removed as vapor through line 23, which may be provided lwith a. liquid dephlegmator 24 for a return of liquid to the bottoms of tower I3.

hour to the top of this column. The vapors leaving the top of the column were passed through I' a-reflux condenser from which reux condensate was returned to the column at `a reflux ratio of 20 `to 1 based on the liquid distillate removed from the column. The liquid phase in thetop of the column thus contained 92 mol percent of solvent which was supplied to the" column at a ratio of 1300 mols of total solvent (water plus salt) per The liquid bot-' 'mo1s of total feed mixture. tomsleaving this column were passed to a second column from which aqueous ethanol was removed as distillate. 93.5% of the ethanol in the feed mixture was recovered in this ysecond distillate product whichv on analysis was found to be free .f of methyl ethyl ketone as 'indicated bya'sensitive .75.

,qualitative test demonstrating the entire absence v tamed@ i f total ofi[carbonli-ls.A The second distillate'productcontained '0 lisopropanl.

Example 3 l f 'Ilhel `ifgflayer' of a' lhvdrocarbolllf :synthesis .product v(froln'carbon monoxide and hydrogen) Vwasolisotllofr'and the followingrraotion obo ef'distillation withitliesalne aqueous sodium'facetat'e solvent in the same column and underthesame operatingfconditions as described above-2in Ex.-

ample 2.Y 'The residue'fromthis column was also distilled .inl 'a' -second'column to Yseparate the `fe'ed components remaining therein lfrom the aqueous sodium acetate. The resulting aqueous "alcohol distillate'4 from the second column ycontained l ethanol and methanol and Was lfoundloy sensitive analysis to be entirely free of bothaldehydessand lietones. Its isopropanol content also was`%..

' Y @The following examples are .also presented to illustratethe advantages in the use of aqueous salt solutions as"the solvent in the vextraotive distillation parooos'sesj offthis invention. Thesev examples involve the' determination of therelaftivo volatllltyof-lsopropy1 alcohol tcl-ethyl alcohol le presence ofl indicated amounts of solvent.

Th l

reux return' conditions insan equilibrium still and malgingfanalyses of the refluxrconden slate A(vapor sample) and the still content (liquid l sample) atl equilibrium conditions. j' The relative volatility thus determined is the volatility of one component dividedy byytllat of the other, the volatility of each component being proportional l to its partial pressure divided bylitsV mol fraction in theliqu'idphase. This is also defined byl the equation liquid fphase'm'olfr'actions of the components to be-separatedjy refersfto the Vapor'phase mol -fraction of these components', subscript'one des- [Was done 'by distilling the mixture underl librium still determinations v, 12 ignates the, more volatile component and subscript two the less volatile component.

Since the relative volatility varies with thev proportion of ethanol to isopropanol, it was del terminecl over a Wide range both with pure water as the solvent and with aqueous sodium acetate (9.73 mol per cent sodium acetate, balance Water). The data, presented in Figure 2, indicate the increase in relative volatility of isopropanoll to ethanol obtained by usingsodium acetate. This increase in Vrelative volatility isV of great practical .valllefas it simplies the ldistillation problem,

,TABLE'III' Y, .'Eect of salts on oapm` liquid equilibria [Liquid composition (salt free) in all cases -5-5 mol percent water lsopropanol and ethanol, respectively-] y Mol Fraction H10 in Vapor Relative Volatility lsopropa'nol 'to Ethanol lillois 'sole per, Liter Salt I ol'.Soll1ti on NH4H1P4 Sodium 'acetate' and disodium acid phosphate (NazI-IPOU have l beenY found .to be particularli7 4eiective saltsboth for increasing the relative volatilit'l7 of isopropanol tof ethanol, for reducing the water content of the vapor and for resisting corrosion to steel'.v iMore vcomplete data oneq'llim l with these salts are presented below-in Tables IV and-Vz' ."TABLEIVl Relative 'volatility',ofv isopropanol over ethanol presence of aqueous sodium acetate solution Relative Volatility of Isopropanol over Ethanol.

concentration in Water with A'rlirsniiiv' i' l' presence of aqueous NazHPOglsolution Mole percentH0 Sodium carbonate in 0.95 mol per cent in Water, when used in distilling a mixture 'of 4 mols isopropanol to 1 mol ethanol with90 mol per cent solvent, gave a relative volatility'of isopropanol to'ethanol of '1.53.v The same salt at 1.52.1'nol per cent concentration in 95 mol per 'cent solvent, when used'in distilling the same alcohol mixture, gave a relative volatility 'of isopropanol to ethanol of 1.55. l

Sodium sulfate'injlA mol percent concentration in solvent, when using `95 mol percent solvent in distilling the` same' alcohol mixture, gave a relative volatility of isoproparn :ll toy ethanol ofifu.;v -r. y

` A'mixture of phosphoric' acid and caustic-soda, giving an empirical formula`(Naalla(PO4)2 o.5, when used in 11.53,; mol per cent concentration in aqueous solvent at 90 mol per jcentfsolvent, in distilling mixtures of ethanol and visopropanol ranging from 90% ethanol to 90% isopropanol, gave relative volatilities of isopropanol to ethanol ranging, respectively, from 1.63 to '1.45.

Ammonium chloride used in 1.53 mol per cent 90 mol per cent solvent, in distilling the same range of mixtures of ethanol and isopropanol, gave relative volatilities of isopropanol to ethanol ranging,"respectively, from 1.73 to 1.41. v

While the abovey examples illustrate changes in the distillation characteristics of aqueous ethanol-isopropanol mixtures caused bythe use of the present invention, theinvention is also applicable to separation of'other close-boiling organic 'mixtures For example, .other alcohols of 2 to about 6 carbon atoms per` molecule such as normal propanol, normal, secondary; tertiary and iso butanols and the primary, tertiary and iso pentanols and hexanols may also be Aseparated by similar distillation methods according to the above described invention. Closeboiling fractions boiling Within the range of about and preferably about 10 to 5 C., containing other classes of oxy organic compounds such asthe aldehydes, ketones, esters, acetals and ketals of about 4 to S carbon atoms per molecule, and hydrocarbons of similar boiling range, may also be separated into classes in the sequence named and into individual compounds in the order of increasing molecular weight oi homologues and of increasing order of isomers by the same methods.

We claim: l

1. The method of separating Water-miscible aliphatic alcohols having 2 to 6 carbon atoms per molecule and which form close-boiling aqueous azeotropes difficult to separate by ordinary fractional distillation in having boiling points which secondary, 1 y

differ by less than 5 C., one of said alcohols hav-,- ing a higher molecular weight than the other, which comprises continuously introducinga iced mixture of said alcohols toa fractionaldistillation zone wherein vapors of said alcohols with Water vapor ascend countercurrently to liquid reflux of the alcohols dissolved in a sufiiciently higher proportion of an aqueous salt solution to effect vaporization of a larger part of the higher molecular Weight alcohol than of the other alcohol in said reflux, continuously introducing an aqueouss'alt solution at an upper part of the fractional distillation zone to maintain atleast 60 mole per cent water in said liquid reiiux and to maintain 1 to 10 mole per cent salt in the Water content of the liquid reflux, continuously removing vapor of the higher molecular Weight alcohol with water vapor overhead from the lfractional distillation zone, and removing af dilute aqueous salt solution of the other alcohol as bottoms., from the fraction distillation zone.

2. A process offfseparating a water-miscible aliphatic alcohol having 2 4to 6 carbonatoms per. molecule from a mixture containing higher molecular weight oxygenated aliphatic organic compound of the class consisting of ketones, aldehydes, esters, ethers, ketals, and acetalsvfwhich have 2 to' 8 carbon atoms per molecule andfwhich are diiicult to separate from the valcohol by' o rdinary fractional distillation, which comprises continuously introducing a feedfmixtur'e of .the

valcohol and said higher molecular weight oxygenated aliphaticv organic "compound to a fractionation zone wherein vapors oifthe vfeed mixture with Water vapor ascend countercurrent1y in contact, with liquid reflux of condensate from said vapors, increasing the Water content of condensates at an upper part of the fractionation zone by introducing an aqueous salt solution continuously thereto sothat the liquid reflux contains at least 60 mole per cent water with about 1 to l0 mole per cent salt dissolved therein to thereby increase the relative volatility of said higher molecular Weight oxygenated organic compound relative to the volatility of lower molecular Weight alcohol present in the feed mixture, continuously removing overhead from said fractional distillation zone vapor of said other higher molecular Weight oxygenated organic compound with water vapor, and continuously withdrawing from a bottom part of the fractionation zone a residual dilute aqueous salt solution containing the lower ymolecular weight alcohol component from the feed mixture substantially freed of said higher molecular weight oxygenated organic component.

3. The method of separating ethyl alcohol from isopropyl alcohol which comprises continuously introducing a feed mixture of said alcohols toV a fractional distillation zone wherein vapors of' ethyl alcohol present in the liquid reflux, with- 'y drawing overhead from the fractional distillation zone vapors of water mixed with alcohol vapors that are `enriched by isopropyl alcohol with respect to the feed mixture, flowing said liquid reflux through a stripping. zone in a lower part'of the fractionation zone countercurrently in contact. with vapors evolved from the liquid reflux, and withdrawing from a. bottom part of the strippingY zone a dilute aqueous salt solution containing ethanolsubstantially freed of isopropanol. l Y Y Y 4. A method according to claim 3 in which said salt is a sodium salt. l

5. A method according to claim 3- in which said salt is a sodium acetate.

6. A. method according to claim 3 in which said salt is" a lithium salt.

7. A 'method according to claim 3 said salt is lithium chloride.

8. A method according to claim 3 said salt-is a magnesium salt.

9. A method according to claim 3 said salt is magnesium chloride.

10. The method of separating `a lower molecular lweight aliphatic alcohol vhaving less than 6" carbon atoms per molecule from a mixture in which inV which in which thereof with a higher molecular weight alcohol having 2 to 6 carbon atoms per molecule and a higher molecular weight aliphatic carbonyl com pound having 2 to 8 carbon atoms per molecule, saidfalcoholsand carbonyll compound forming close-boilingA azeotropes dimcult to separate by ordinaryfractional distillation in having'boiling points which differ by less than C., Whichcomvprises heating an aqueous salt solution of said mixture in a'iirst fractionation zone to evolve vapor therefrom, passing an aqueous salt solution downwardly through said fractionation zone 'countercurrently to said evolved vapors to maintain in reiiuxing liquid phases existing throughout said fractionation'zone above about 85 mole V enhanced proportion of higher molecular weight alcoholjand carbonyl compound with respect to the lower molecular weight alcohol in the feed mixture, and withdrawing from a bottom part of said fractionation zone a dilute aqueous salt solution enhanced in the proportion ofthe lower molecular weight alcohol with respect to the higher molecular weight alcohol and carbonyl compound present in the feed mixture composition. f

11. 'Ivhe process according to claim 10, in which said dilute aqueous salt solution withdrawn from the bottom of the fractionation zone is passed to a second distillation zone, and the lower molecular weight alcohol is distilled from the aqueous salt solution substantially free of higher molecular weight organic components present in the feed mixture.'y Y

l2. The process according to claim 10, in which salt solution residue from said second distillation zone is returned to an upper part of the first fractionation zone.

13. The process according to claim 10, in which vapors rising from the rst fractionation zone pass upwardly through a second rectification zone countercurrentr to descending liquid reiiux containing condensate of vapors leaving the top of second rectification zone, and in which distillate obtained from the rectification zone is thus reduced in water content to a composition approximating that `of aqueous azeotrope` of organic compounds passed upwardly through the second rectification zone.

14. A process according to claim 4 in which said sodium salt is disodium hydrogen phosphate.

. vCHARLES E. MORRELL.

EDWIN R'. GILLILAND.

. REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS MetZl July 21, 1942 

1. THE METHOD OF SEPARATING WATER-MISCIBLE ALIPHATIC ALCOHOLS HAVING 2 TO 6 CARBON ATOMS PER MOLECULE AND WHICH FORM CLOSE-BOILING AQUEOUS AZEOTROPES DIFFICULT TO SEPARATE BY ORDINARY FRACTIONAL DISTILLATION IN HAVING BOILING POINTS WHICH DIFFER BY LESS THAN 5* C., ONE OF SAID ALCOHOLS HAVING A HIGHER MOLECULAR WEIGHT THAN THE OTHER, WHICH COMPRISES CONTINUOUSLY INTRODUCING A FEED MIXTURE OF SAID ALCOHOLS TO A FRACTIONAL DISTILLATION ZONE WHEREIN VAPORS OF SAID ALCOHOLS WITH WATER VAPOR ASCEND COUNTERCURRENTLY TO LIQUID REFLUX OF THE ALCOHOLS, DISSOLVED IN A SUFFICIENTLY HIGHER SALT SOLUTION AT AN UPPER PART OF THE EFFECT VAPORIZATION OF A LARGER PART OF THE HIGHER MOLECULAR WEIGHT ALCOHOL THAN OF THE OTHER ALCOHOL IN SAID REFLUX, CONTINUOUSLY INTRODUCING AN AQUEOUS SALT SOLUTION AT AN UPPER PART OF THE FRACTIONAL DISTILLATION ZONE TO MAINTAIN AT LEAST 60 MOLE PER CENT WATER IN SAID LIQUID REFLUX AND TO MAINTAIN 1 TO 10 MOLE PER CENT SAID IN THE WATER CONTENT OF THE LIQUID REFLUX, CONTINUOUSLY REMOVING VAPOR OF THE HIGHER MOLECULAR WEIGHT ALCOHOL WITH WATER VAPOR OVERHEAD FROM THE FRACTIONAL DISTILLATION ZONE, AND REMOVING A DILUTE AQUEOUS SALT SOLUTION OF THE OTHER ALCOHOL AS BOTTOMS FROM THE FRACTION DISTILLATION ZONE. 